1. The Field of the Invention
The invention relates to a radiation-selective absorber coating, in particular for an absorber tube of a parabolic trough collector, which comprises a reflective layer that reflects in the infrared range, at least one barrier layer arranged below the reflective layer, at least one absorption layer arranged above the reflective layer and an antireflection layer arranged above the absorption layer. The invention also relates to an absorber tube that has the aforesaid radiation-selective coating, and to a method of operating a parabolic trough collector using this sort of absorber tube. Parabolic trough collectors are used for generating power in solar power plants.
2. The Description of the Related Art
Customary absorber coatings consist of a layer which is reflective in the infrared range and is applied on a substrate, in particular a metal tube, and also a cermet layer having a high absorptance in the range of the solar spectrum and a covering layer applied on the cermet layer, which covering layer is referred to as an antireflection layer and, owing to the high refractive index of the cermet layer, is provided for reducing the surface reflection on the cermet layer.
A fundamental endeavor is to achieve an energy yield that is as high as possible. The energy yield is dependent, inter alia, on the coefficients of the absorptance α and the emissivity ε. Efforts are focussed on obtaining an absorber coating with a high absorptance (α≥95%) and a low emissivity (ε≤10%).
Furthermore, the efficiency of a solar power plant is determined by the operating temperature of the collector array. From this standpoint, a temperature that is as high as possible is desired. Contrary to this, the durability of the layer system of the absorber coating decreases, however, with increasing operating temperature on account of ageing and/or diffusion processes, as a result of which, for example, the absorption property of the cermet layer and the reflection property of the reflective layer that reflects in the infrared range can decline significantly.
Molybdenum is usually used for the reflective layer that reflects in the infrared range. However, the reflection properties of a molybdenum layer are not optimal, and so it is desirable to use better reflective materials. Therefore, other materials having better IR reflection properties such as copper or silver are also used for the reflective layers which are reflective in the IR range.
The operating temperature of known absorber tubes is 300-400° C. For the reasons above, what is fundamentally striven for is to increase the operating temperature further, but without impairing, for example, the absorption properties of the cermet layer and the reflection properties of the reflective layer that is reflective in the infrared range.
Such endeavors are summarized in C. E. Kennedy, “Review of Mid- to High-Temperature Solar Selective Absorber Materials”, Technical Report of the National Renewable Energy Laboratory, July 2002 edition. This reference discloses a layer construction composed of a ZrOxNy or ZrCxNy absorption layer and a layer of Ag or Al which is reflective in the IR range. This layer structure has improved thermal stability in air by virtue of the introduction of an Al2O3 diffusion barrier layer. It was furthermore ascertained that the thermal stability of the infrared reflection layer under reduced pressure can be improved by the introduction of a diffusion barrier layer below this layer. For this barrier layer, Cr2O3, Al2O3 or SiO2 are proposed as the layer material. The hope is to achieve stability of the silver reflection layer up to 500° C.
However, this does not end the striving for more durable layers in conjunction with still good absorptance and emissivity.
Therefore, DE 10 2006 056 536 A1 describes a radiation-selective absorber coating comprising at least two barrier layers, a layer which is reflective in the IR range and is arranged thereon, an absorption layer arranged above the reflective layer, and an antireflection layer arranged above the absorption layer, which have a high solar absorptance and a low thermal emissivity.
Although the adhesion of the IR-reflective layer, which preferably consists of silver, is sufficient, it is still in need of improvement. In particular, it has been found that the substrate pretreatment in the production process has a great influence on the layer adhesion. Thus, the layer adhesion can be adversely influenced for example by relatively long storage times or external influences, such as moisture or particle entry prior to coating.
DE 20 2006 009 369 U1 describes a composite material for a solar collector element which contains, in its layer system, a separating layer comprising at least one partial layer consisting of plastic.